1. Field of the Invention
The present invention relates to an optical disc reproduction apparatus and method, and more particularly, to an optical disc reproduction apparatus and method for correcting a signal equalization characteristic of an equalizer using an output of a phase locked loop.
Since an optical disc such as a compact disc (CD) or a digital video disc (DVD) has the merits of a large capacity of recording data owing to a high density of the recording data as compared with a conventional magnetic recording medium, and a relatively fast access time, the optical disc is variously employed in multimedia products or their applications. However, such an optical disc has a considerably slow operation speed, as compared with a hard disk drive or a semiconductor memory to be used as a peripheral device of a personal computer (PC). Accordingly, a variety of methods for obtaining an access time required when an optical disc is used as a peripheral device of the computer have been attempted.
2. Description of the Related Art
In a constant linear velocity (CLV) reproduction method used for heightening a density of recording data on the optical disc, the rotational amount of an optical disc required when data to be read is positioned in the innermost circumference of the optical disc is increased by a factor of two, in comparison with the case when data to be read is positioned in the outermost circumference thereof. However, it takes a much longer time to vary the rotational amount of an optical disc so that the disc has a rotational amount appropriate for reading out data from a target position, rather than to move the pick-up to the target position of the disc. Thus, to reduce the time required for variation of the rotational amount of the disc a method of reducing an access time is used.
As a method for reducing the time required for variation of the rotational amount of the disc, a method for enhancing performance of a spindle motor has been considered. But there is a drawback that an optical disc reproduction apparatus increases in cost. Another method used is a wide-capture range phase-locked loop (hereinafter, refereed as a "wide PLL") for allowing data to be obtained within several tens of % (or above) of a desired rotational amount. A radio frequency (RF) signal which is a high frequency signal read from the optical disc is modulated to serve as self-clock recovery. Therefore, if a PLL which is used to convert the signal read from the optical disc into a binary signal has a broad clock recovery range, data reproduction can be done without converging the rotational amount of the disc to a normal velocity. Accordingly, use of the wide PLL provides a fast access time. The performance of the wide PLL determines a range capable of variable velocity reproduction.
Meanwhile, in the optical disc reproduction apparatus, there is an optical transfer function determined by a wave length of a laser beam, a numerical aperture (NA) of an objective lens, the minimum length of a pit, etc., and an allowable range of reproduction velocity is limited by the optical transfer function. A reproduced signal involves deterioration of the signal. An equalizer is used for correcting the deterioration of the signal. Since a CD relatively stabilizes against the limitation of reproduction velocity by the optical transfer function and the deterioration of the signal, an equalizer is not necessarily required for speed reproduction. However, when a CD is reproduced at a high speed to heighten a transmission rate of data, performance is improved by use of the equalizer. In the case of a DVD, the equalizer is necessarily required.
FIG. 1A shows part of a signal reproducer of a conventional optical disc reproduction apparatus. The apparatus of FIG. 1A comprises a wide PLL 12 and an equalizer 11 having a fixed characteristic value optimized for a constant linear velocity reproduction. FIG. 1B shows a linear velocity and jitter characteristic of the equalizer 11 which is designed to be optimized for constant linear velocity reproduction. When reproducing a CD at a constant linear velocity, the equalizer 11 having the fixed characteristic value can generate a signal deteriorated by the wide PLL 12, but the equalizer 11 can be used without any correction of its characteristic because the degree of deterioration due to the wide PLL 12 is minor. However, it is not possible to obtain the optimized equalization characteristic in variable velocity reproduction due to the fixed characteristic value of the equalizer 11. When the wide PLL 12 is used for reproducing the DVD, the limitation of reproduction velocity and deterioration of a signal occur, and thus the characteristic of the equalizer should be necessarily corrected.